1. Field of the Invention
The present invention relates to an optical pickup apparatus that executes an operation of reading a signal recorded in an optical disc or an operation of recording a signal in the optical disc with laser light.
2. Description of the Related Art
Optical disc devices has been widespread each of which is capable of a signal reading operation and signal recording operation by applying laser light emitted from an optical pickup apparatus to a signal recording layer of the optical disc.
The optical disc devices using optical discs called CDs or DVDs are available in general, however, optical discs whose recording densities are improved, that is, those using Blu-ray standard optical discs have recently been developed.
Infrared light with a wavelength of 780 nm is used as the laser light executing the operation of reading a signal recorded in a CD standard optical disc, and red light with a wavelength of 650 nm is used as the laser light executing the operation of reading a signal recorded in a DVD standard optical disc.
In contrast to cases of such CD-standard and DVD-standard optical discs, laser light with a short wavelength, or a blue-violet light with a wavelength of 405 nm, for example, is used as the laser light executing the operation of reading a signal recorded in a Blu-ray standard optical disc.
The thickness is 0.1 mm of a protective layer provided on an upper surface of the signal recording layer in the Blu-ray standard optical disc, and the numerical aperture is specified at 0.85 of an objective lens used for the operation of reading a signal from this signal recording layer.
For such an optical pickup apparatus compliant with the optical disc standard with improved recording density, strict optical characteristics are required to improve signal recording quality in accordance with improvement in the recording density.
The optical pickup apparatus is made up such that a driving current to be supplied to a laser diode can be controlled so as to be able to obtain a laser output suitable for reading a signal recorded in the optical disc or a laser output suitable for recording a signal in the optical disc. The above operation of controlling the laser output is performed such that a monitor signal, which is obtained from a photodetector for front monitor provided at a position where the laser light emitted forward from the laser diode is applied, is fed back to a driving circuit for supplying the driving current to the laser diode, as known.
If glass is used as material of an objective lens for focusing the laser light to the signal recording layer included in the optical disc, since it is not affected by temperature, signal recording characteristics and signal reproducing characteristics can be improved, however, there is a problem of high cost. As a method for solving such a problem, a method is generally performed of manufacturing an objective lens by injection-molding a synthetic resin.
The objective lens made of a synthetic resin has a problem that spherical aberration and chromatic aberration occur due to a change in temperature, and especially when the laser output is greatly changed as in the case of switching from a recording operation state to a reproducing operation state, a problem called mode hop occurs, which is a problem that a focal position of the objective lens is moved in response to a change in wavelength of the laser light, so that the focal point is deviated.
Such a phenomenon of deviation of the focal position is mainly caused by aberration called chromatic aberration which occurs in the objective lens due to a change in wavelength of the laser light, and when the focal position is deviated due to such a cause, a problem occurs that the operation of reproducing a signal recorded in the optical disc or the like cannot normally be performed. As a method for solving such a problem, a technology has been developed for improving the chromatic aberration by forming a diffraction grating at the objective lens (See Japanese Patent Laid-Open Publication No. 9-311271).
The above-mentioned Patent Document discloses the technology to correct the chromatic aberration by providing a diffraction grating at the objective lens, and in the optical pickup apparatus using the objective lens with a large numerical aperture such as the optical pickup apparatus for the Blu-ray standard optical disc, the objective lens with a small radius of curvature is used.
If a diffraction grating such as an annular diffraction grating is formed at an objective lens with a small radius of curvature, transmittance of the laser light is reduced, so that the intensity becomes insufficient of the laser light applied to the signal recording layer of the optical disc, which is a problem. Considering such reduction in laser light intensity, the size of a driving signal to be supplied to the laser diode needs to be increased, and thus, the heat generated from the laser diode is further increased so that the spherical aberration and chromatic aberration of the objective lens are increased, which is a problem.